The present invention relates to encoding control of a motion video signal encoding apparatus for use in television conference systems and television-utilizing education systems, and more particularly to an encoding apparatus for controlling the coarseness of a quantization characteristic to keep substantially constant the quantity of information generated after encoding.
A motion video signal encoding system, or a data compression system, using interframe coding or the like for information source coding, subjects input motion video signals to interframe prediction, and obtains prediction error signals. These prediction error signals are quantized by a quantizer into quantized prediction error signals. These quantized prediction error signals are converted by a variable word length coder into a variable word length code sequence to be written into a buffer memory. The contents of this buffer memory are read out at a constant transmission rate, and sent out to a transmission path. This buffer memory performs rate matching between the variable word length coder output, which output at random timings, with the constant transmission rate on the transmission path.
The average output code length of this variable word length coder varies dependent on the area of the moving parts contained in the motion video picture. Therefore, since the average code length of the output of the variable word length coder is extended when motion video pictures having a larger moving part area are consecutively entered, the buffer memory is exposed to the risk of overflow. In order to prevent the buffer memory from overflowing, when the area of moving parts in motion video signals grows, i.e. the average code length of the output of the variable word length coder extends, beyond a certain limit, the encoding apparatus reduces the average code length of the output of the variable word length coder by making the quantization characteristic of the quantizer coarser. The cost of this reduction in output code quantity is a deterioration in the quality of decoder pictures reproduced on the receiving side. Such control will be referred to as encoding control hereinafter in this specification.
For an example of such encoding control, reference may be made to Y. Takishima and M. Wada, "A Study on control Method for Low Bit Rate Video Coding" in the 1990 Spring National Convention Record, The Institute of Electronics, Information and Communication Engineers, lecture No. D-311.
By the encoding control technique described in this reference, the number of codes in the output of a variable word length coder is counted for every fixed period, for instance one frame period; the quantization characteristic of sufficient coarseness to reduce this count to a prescribed number of codes per-frame is estimated according to the count and to the quantization characteristic used in this frame time; and this estimated quantization characteristic is used for encoding the motion video signals of the next frame.
The encoding control method described in this reference, however, is susceptible to the following disadvantage when a moving object occupying a large area in the television frame suddenly stops. For instance, suppose that this moving object is moving until frame No. 99, and stops in frame No. 100. The quantization characteristic to be used for encoding the picture of frame No. 100, which is determined by the count obtained for the picture of frame No. 99, will be coarse. Then, because this coarse quantization characteristic is used for the encoding of the picture of frame No. 100 though the previously moving large object in it is now static, the count for the picture of frame No. 100 will become smaller. As fine quantization characteristic is to be used for the encoding of the picture of frame No. 101, the count for this picture will become greater again. As a consequence, a coarse quantization characteristic is used for the encoding of the picture of frame No. 102. Thus, according to the prior art described in this reference, if an object of a large area suddenly stops, coarse and fine quantization characteristics may be selected for alternate frames. In this case, a decoded picture for which the coarse quantization characteristic was used would involve a large encoding distortion, while one for which the fine quantization characteristic was used would be substantially free from encoding distortion, so that the observer at the receiving end would be shown decoded pictures alternately greatly distorted and hardly distorted, varying from frame to frame. This alternate presentation of greatly distorted and hardly distorted pictures would give the observer an impression of a greater deterioration in picture quality than when greatly distorted pictures alone are presented.